Proportion Question on Percentages in Ripe Starters

I have been maintaining my mother starter for a few months now - I am new at this sourdough 'thing' - and am ready to plunge into the 'why' question.

I have been following Peter Reinhart's suggested proportions when I do my weekly feeds.  I start with 30g starter and add 90g flour and 67g water (75% hydration). I let it sit out for a bit then toss it into the refrig. until ready to use at which time I feed it a couple of times the day before baking with it.  My first feed is 15g:15g:15g and the subsequent feeds go to the formula in Reinhart's book.

My question is why am I using less starter after the initial feed of 15g:15g:15g.  For instance, I am experimenting with my starters now - watching them double etc and I am practicing just eyeballing what I add to my batch.  Mixing it so I am doubling what I have left in the container.  This morning I decided to see how accurate my eyeballing is and I was pretty close but my proportions were 50g:50g:50g....The starter amount equal to both the flour and water I am adding.  

From what I have read here starter amounts on builds are generally a lot less than what gets added to it.  

Why is that?  

Thanks